English

Noun

In ecology, predation describes a
biological
interaction where a predator organism feeds on another living
organism or organisms known as prey. Predators may or may not kill
their prey prior to feeding on them, but the act of predation
always results in the (ecologically significant) death of the prey.
The other main category of consumption is detritivory, the consumption
of dead organic material (detritus). It can at times be
difficult to separate the two feeding behaviors

Degree of specialization

Among predators there is a large
degree of specialization. Many predators specialize in hunting only
one species of prey. Others are more opportunistic and will kill
and eat almost anything (examples: humans, leopards, and dogs). The specialists are usually
particularly well suited to capturing their preferred prey. The
prey in turn, are often equally suited to escape that predator.
This is called an evolutionary
arms race and tends to keep the populations of both species in
equilibrium. Some predators specialize in certain classes of prey,
not just single species. Almost all will switch to other prey (with
varying degrees of success) when the preferred target is extremely
scarce, and they may also resort to scavenging or a herbivorous
diet if possible.

Trophic level

Predators are often another organism's prey, and
likewise prey are often predators. Though blue jays prey
on insects, they may in
turn be prey for snakes,
which may themselves be the prey of hawks. One way of classifying
predators is by trophic
level. Organisms which feed on autotrophs, the producers of
the trophic
pyramid, are known as herbivores or primary
consumers; those that feed on heterotrophs such as animals
are known as secondary consumers. Secondary consumers are a type of
carnivore, but there
are also tertiary consumers eating these carnivores, quartary
consumers eating them, and so forth. Because only a fraction of
energy is passed on to the next level, this hierarchy of predation
must end somewhere, and very seldom goes higher than five or six
levels. A predator at the top of any food chain
(that is, one that is preyed upon by no organism) is called an
apex
predator; examples include the orca, tiger, and crocodile and even omnivorous
humans. An apex predator in one environment may not retain this
position if introduced to another habitat, such as dogs among
crocodilians.

The problem with this system of classification is
that many organisms eat from multiple levels of the food chain. A
carnivore may eat both secondary and tertiary consumers, and its
prey may itself be difficult to classify for similar reasons.
Organisms showing both carnivory and herbivory are known as
omnivores. Even
supposedly strict herbivores may supplement their diet with meat.
Carnivorous
plants would be very difficult to fit into this classification,
producing their own food but also digesting anything that they may
trap. Organisms which eat detritivores would also be
difficult to classify by such a scheme.

Ecological role

Predators may increase the biodiversity of communities
by preventing a single species from becoming dominant. Such
predators are known as keystone
species, may have a profound influence on the balance of
organisms in a particular ecosystem. Introduction or
removal of this predator, or changes in its population density, can
have drastic cascading effects on the equilibrium of many other
populations in the ecosystem. For example, grazers of a grassland
may prevent a single dominant species from taking over.

Adaptations and behavior

The act of predation can be broken
down into a maximum of four stages: Detection of prey, attack,
capture and finally consumption. The relationship between predator
and prey is one which is typically beneficial to the predator, and
detrimental to the prey species. Sometimes, however, predation has
indirect benefits to the prey species, though the individuals
preyed upon themselves do not benefit. This means that, at each
applicable stage, predator and prey species are in an evolutionary
arms race maximize their respective abilities to obtain food or
avoid being eaten. This interaction has resulted in a vast array of
adaptations in both
groups.

General

One adaptation helping both predators and prey
avoid detection is camouflage, a form of
crypsis where species
have an appearance which helps them blend into the background.
Camouflage consists of not only color, but also shape and pattern.
The background upon which the organism is seen can be both its
environment (e.g. the praying
mantis to the right resembling dead leaves) other organisms
(e.g. zebras' stripes
blend in with each other in a herd, making it difficult for
lions to focus on a single
target). The more convincing camouflage is, the more likely it is
that the organism will go unseen.

While successful predation results in a gain of
energy,
hunting invariably involves energetic costs as well. When hunger is not an issue, most
predators will generally not seek to attack prey since the costs
outweigh the benefits. For instance, a large predatory fish like a
shark that is well fed in
an aquarium will
typically ignore the smaller fish swimming around it (while the
prey fish take advantage of the fact that the apex predator is
apparently uninterested). Surplus
killing represents a deviation from this type of behaviour. The
treatment of consumption in terms of cost-benefit
analysis is known as optimal
foraging theory, and has been quite successful in the study of
animal
behavior. Costs and benefits are generally considered in energy
gain per unit time, though other factors are also important, such
as essential
nutrients that have no caloric value but are necessary for
survival and health.

Size-selective predation involves predators
preferring prey of a certain size. Large prey may prove troublesome
for a predator, while small prey might prove hard to find and in
any case provide less of a reward. This has led to a correlation
between the size of predators and their prey. Size may also act as
a refuge
for large prey, for example adult elephants are generally safe from
predation by lions, but juveniles are vulnerable.

Mobbing can be an interspecies activity: it is
common for birds to respond to mobbing calls of a different
species. Many birds will show up at the sight of mobbing and watch
and call, but not participate. It should also be noted that some
species can be on both ends of a mobbing attack. Crows are
frequently mobbed by smaller songbirds as they prey on eggs and
young from these birds' nests, but these same crows will cooperate
with smaller birds to drive away hawks or larger mammalian
predators. On occasion, birds will mob animals that pose no
threat.

Black-headed
Gulls are one species which aggressively engages intruding
predators, such as Carrion
Crows. Experiments on this species by Hans Kruuk involved
placing hen eggs at intervals from a nesting colony, and recording
the percentage of successful predation events as well as the
probability of the crow being subjected to mobbing. The results
showed decreasing mobbing with increased distance from the nest,
which was correlated with increased predation success. Mobbing may
function by reducing the predator's ability to locate nests, as
predators cannot focus on locating eggs while they are under direct
attack.

Advertising unprofitability

Once a predator has detected
its prey, one would expect it to pursue it. However, it is not
always profitable for the predator to do so. Consider the example
of a Thomson's
Gazelle being spotted by a predator. Giving chase to prey
requires a sacrifice in energy. If, however, there is some way the
prey species can convey the information that it is unprofitable,
energy will be saved by both organisms. Thomson's Gazelles are
hunted by species such as lions and cheetahs. When they see the
predator approach, they may start to run away, but then slow down
and stot. Stotting describes
a behavior involving jumping into the air with the legs kept
straight and stiff, and the white rear fully visible. Obviously
this behavior is maladaptive if they hope to outrun the predator,
so it must serve some other purpose. Although other hypotheses
have been put forward, evidence supports the proposition that they
stot to signal
an unprofitable chase. For example, cheetahs abandon more hunts
when the gazelle stots, and in the event they do give chase, they
are far less likely to make a kill.

Aposematism,
where organisms are brightly colored as a warning to predators, is
the antithesis of camouflage. Some organisms pose a threat to their
predators - for example they may be poisonous, or able to harm them
physically. Aposematic coloring involves bright, easily
recognizable and unique colors and patterns. Upon being harmed
(e.g. stung) by their prey, the appearance of such an organism will
be remembered as
something to avoid.

Population dynamics

It is fairly clear that predators tend
to lower the survival and fecundity of their prey, but
on a higher level of organization, populations of predator and
prey species also interact. It is obvious that predators depend on
prey for survival, and this is reflected in predator populations
being affected by changes in prey populations. It is not so
obvious, however, that predators affect prey populations. Eating a
prey organism may simply make room for another if the prey
population is approaching its carrying
capacity.

Humans and predation

In conservation

Predators are an important consideration in
matters relating to conservation.
Introduced
predators may prove too much for populations which have not
coevolved with them,
leading to possible extinction. This will depend
largely on how well the prey species can adapt to the new species,
and whether or not the predator can turn to alternative food
sources when prey populations fall to minimal levels. If a predator
can use an alternative prey instead, it may shift its diet towards
that species in a behavior known as functional
response, while still eating the last remaining prey organisms.
On the other hand the prey species may be able to survive if the
predator has no alternative prey - in this case its population will
necessarily crash following the decline in prey, allowing some
small proportion of prey to survive. Introduction of an alternative
prey may well lead to the extinction of prey, as this constraint is
removed.

Predators are often the species endangered
themselves. Competition
for prey from other species could prove the end of a predator - if
their ecological
niche overlaps completely with that of another the
competitive exclusion principle requires only one can survive.
Loss of prey species may lead to coextinction of their
predator. In addition, because predators are found in higher
trophic levels, they are less abundant and much more vulnerable to
extinction.

Biological pest control

Predators may be put to use in
conservation efforts to control introduced species. Although the
aim in this situation is to remove the introduced species entirely,
keeping its abundance down is often the only possibility. Predators
from its natural range may be introduced to control populations,
though in some cases this has little effect, and may even cause
unforeseen problems. Besides their use in conservation
biology, predators are also important for controlling pests in
agriculture. Natural
predators are an environmentally friendly and sustainable way of
reducing damage to crops, and are one alternative to the use of
chemical agents such as pesticides.